Selective signaling system



A. E. BACHELET ET AL SELECTIVE SIGNALING SYSTEM Filed Sept 15, 1956 FIG.

April 4, 1939.

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I 90 |80 NUMEER OF DEGREES BY WHICH GRID VOLTAGE LAGS BEHIND ANODEVOLTAGE PHASE SHIFT/I16 Patented Apr. 4, 1939 PATENT OFFICE SELECTIVESIGNALING SYSTEM Albert E. Bachelet, Mount Vernon, and William H. T.Holden, New York, N. Y.; said Bachelet assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New York,and said Holden assignor to American Telephone and Telegraph Company, acorporation of New York Application September 15, 1936, Serial No.100,848

Claims.

This invention relates to selective signal receiving circuits.

An object of the invention is to provide an arrangement of the abovecharacter which will respond to a single unmodulated frequency anddiscriminate sharply against all other frequencies.

A feature of the invention whereby the above object is obtained residesin the employment of a discharge device of the so-called gas-filled ortrigger type in conjunction with a nonvariable phase shifting network soconstructed as to effect a 180 degree shift in phase in the incomingwave at a predetermined critical frequency.

More specifically, one embodiment of the invention resides in areceiving arrangement for supervision, ringing, and other signalingpurposes, over telephone lines and the like which employs alternatingcurrent of a frequency within the voice range comprising a phase shiftnetwork, a gas-filled discharge device, an amplifier, a full-waverectifier, a guard relay, and a signal relay, all so arranged andconnected that at all frequencies other than said predetermined criticalor signaling frequency the gas-filled device will be ionized and bothsaid relays operated thereby closing the signal circuit at one point andopening it at another and at said critical frequency the gas-filleddevice will be deionized and one of said relays released therebycompleting the signal circuit.

The present arrangement which employs the phase shift-frequencycharacteristic of a network has a considerable advantage over previoussystems employing resonant circuits or bandpass filters which dependupon an attenuation frequency characteristic.

To secure sharp discrimination between frequencies differing by a smallpercentage, on the latter basis, requires the use of low loss coils andcondensers having a high ratio of reactance to resistance. Such coilsare of necessity large and expensive. However, by utilizing the phaseshift-frequency characteristic it becomes possible to employ coils ofrelatively high resistance because of the sudden change in current whenthe relative grid and anode phase shift a few degrees past 180 degrees.

The invention will be understood from the following description whenread in connection with the accompanying drawing.

Fig. 1 shows a signaling system in accordance with the above-mentionedspecific embodiment employing a phase shift network and a singlegasfilled discharge device arranged to display a signal only whenalternating current is being transmitted over the line at thepredetermined critical frequency alone or in combination with otherfrequencies.

Fig. 2 shows another embodiment of the invention in which two gas-filleddevices are used together with a phase shifting network so arranged thatthe control electrode of one device is subjected to the frequencyflowing in the line without phase displacement, whereas the othercontrol electrode is subjected to a wave which has been displaced inphase with respect to the other control electrode supply by passing theline frequency through a phase shifting network. In this case the outputof the phase network is reversed or transposed in order that when onewave is displaced 180 degrees with respect to the other any twosimultaneous half waves will be of the same sign and hence both controlelectrodes will at any instant be at the same potential.

Fig. 3 graphically shows the per cent of maximum average anode currentpassed by the gasfilled tube of Fig. 1 at different degrees by which thegrid voltage lags behind the anode voltage.

Fig. 3 will be understood by referring to an article by Hull in theGeneral Electric Review for July 1929, pages 391 to 393, which describesa general method of operating a gas discharge device (thyratron) with analternating anode voltage and a method of controlling the averageoutput, or anode current, thereof by varying the phase of the gridvoltage with respect to that of the anode. Referring to Fig. 28 on page393 of the foregoing publication, it will be evident that a fourth case(d) could be assumed in which the phase of the grid voltage, withrespect to the anode voltage, is further shifted to a little more than180 degrees whereupon current starts at the beginning of each cycle andconsequently 100 per cent maximum average current flows each half cycle,this conditon continuing as the phase of the grid voltage is shiftedtowards 360 degrees at which point, however, the maximum average beginsto decrease until it reaches a point, 180 --q at which point little orno current flows, angle 1 depending on the minimum ionizing potentialrequired for the anode.

A more complete understanding of the invention may be obtained from thefollowing description which discusses in detail each operating step fromthe receiving of the incoming current signal to the operation of therelay that lights the signal lamp.

Referring now to the arrangement of Fig. 1, the line Ll is connected tothe left winding of transformer I, the right winding of which is inseries with the circuit of grid element 2 of the electronic amplifyingtube 3. Line Ll is also connected to a phase shifting network 4, whichis represented conventionally by a box without showing details sincethis type of apparatus is Well-known in the art, it only being necessaryto state that the type of network is one which exhibits little or noattenuation to current supplied thereto within the predetermined rangeof frequencies transmitted over the line and which causes a displacementof the phase of waves within a critical range of said transmittedfrequencies of degrees, and that furthermore this phase shift iscontinuously variable with change in frequency. Network 4 is connectedto the left winding of transformer 5 the right winding of which isconnected in the circuit of grid element 6 of ionic gas-filled tube 1,which is of the so-called hot cathode type and is so constructed thatthe grid circuit can control, in general, only the starting of the anodecurrent. Except for low anode currents the grid element of this type ofionic tube ordinarily loses control once the anode current is started.The anode element 3 of the amplifying tube 3 is in series with the leftwinding of transformer 9 and battery Ill. The upper right Winding oftransformer 9 is connected as shown to a full wave rectifier H which maybe of the well-known copper-oxide type, or any other type with suitablecharacteristics. The output of this rectifier is connected to thewinding of a line relay 12. The lower right winding of transformer 9 isin series with the winding of guard relay H! in the anode-cathodecircuit of ionic gas-filled tube 1,

Under the normal non-signaling condition on line Ll, the frequency ofthe incoming current is such that the current delivered by network 4 tothe left winding of transformer 5 lags in phase behind the currentdelivered to the left winding of transformer l by angles from 180degrees to 360 degrees and from zero to 180 degrees I where I is a smallangle that depends on the minimum ionizing potential required for theanode circuit of the tube. For all angles of lag from 180 degrees to 360degrees it has been found that the average current through theanode-cathode circuit of tube I, which includes the winding of relay I4,is maintained at a constant maximum; for angles from'zero to an angleless than 180 degrees by the small angle I aforesaid, the anode-cathodecurrent gradually decreases from its maximum until it reaches zero. Thisis illustrated graphically by the curve of Fig. 3. During thisnon-signaling condition relays l2 and M are both continuously operatedand consequently no circuit is closed through lamp I6, which remainsunlighted.

To signal over the line Ll, current of a predetermined criticalfrequency, varying over only a small range is connected to line Ll. Thisfrequency is so determined that the current delivered to the gridcircuit of tube 1, which current is limited by the series resistance 24,differs in phase from that delivered to the anode circuit of tube I byan angle within the range of 180 degrees and 180 degrees I the angle ibeing as aforesaid. Under this condition no current flows in the anodecircuit of tube 1, relay [4 releases and a circuit is closed from groundon the contacts of relay I4, through operated contacts of relay I2 tobattery, through lamp [6, which lights. Relay I2 is operatedcontinuously while current is applied to line LI, since it is energizedby the full wave rectifier l I. As a feature of this invention,

when there is no current on line Ll, both relays l2 and. M are thenreleased, and the circuit through lamp I6 is open.

Referring now to the arrangement of the invention shown by Fig. 2, theline L2 is connected through the phase shifting network I? and thereversed conductors l8 and I!) to the left winding of the transformer 20and directly to the left winding of the transformer 2|. The rightwindings of these transformers are connected in the grid circuits of thegas-filled ionic tubes 22 and 23, which are of the hot cathode type andare similar to the tube 1 of Fig. 1. To limit the flow of current in thegrid circuits resistances 31 and 38 are inserted. Normally tubes 22 and23 are deionized due to the negative bias supplied to the grid elements28 and 29 by the biasing batteries 39 and 40. The anode-cathode circuitsof tubes 22 and 23 are serially connected together by the conductor 25and are in series with the winding of line relay 2'6 and the localsource of alternating current 21, which current should differ infrequency from that of the signaling current applied to line L2, thereason for which will be hereinafter given. Under the normalnon-operating condition, the shifting of the phase of the incomingcurrent due to the network l'-! and its reversal over conductors l8 and[9 results in a difference in phase of the potentials applied to thegrid elements 28 and 29. Accordingly the tubes 22 and 23 can ionizesimultaneously for only apart of the positive half-cycle of the incomingcurrent, and this time of simultaneous" ionization is arranged to be sosmall that the average current that flows through the winding of relay26, during the positive half-cycle of the current from source 21 isinsufficient to operate relay 26. If the frequency of the incomingcurrent should be such that there is no shifting of phase due to thepassage through the network I1, then the reversal over conductors l5 andIE will cause the potentials of grid elements 28 and 29 to differ inphase by 180 degrees and under this condition there will be nosimultaneous ioniza'- tion of tubes 22 and 23.

When signals are transmitted over a telephone circuit, as for example inthe well-known 1000 cycle ringing system employed on repeater circuit,it is necessary to use frequency selective means to discriminate betweensuch signals and voice frequency currents from other sources. This isaccomplished in part by tuning and in part by a low frequency modulationof the 1000' cycle current. Very sharply resonant circuits areunsatisfactory for this purpose owing to the fact that the sidefrequencies resulting fromthe modulation above mentioned may be excludedor attenuated. Furthermore, a. circuit having very sharp tuning wouldexhibit large variations in response to small shifts in the signalingfrequency which would result in a material increase in the cost of thesignaling current supply. For these reasons it has been found preferableto utilize frequency selecting means having band pass characteristicsand exhibitingv a uniform response to all frequencies within apredetermined band but greatly attenuating all frequencies outside thisband. The arrangements of this invention accomplish this, sinceoperation of the signal will result for all frequencies such that thephase shift due to the network is within a range of i degrees extendingeither side 180, wherein 1 depends on the minimum ionizing potentialrequired by the anode. How.- ever, in. some: cases a wider band thanthis may be desired and network I! may be so designed as to have a phasefrequency characteristic adapted to produce operation of the signaldevice over the frequency range required by the particular signalingsystem to which the invention is applied. This may be accomplished byutilizing a plurality of network circuits having a constant phase shiftover a range of frequencies and so combined that the resultant phaseshift will be constant at degrees over a predetermined band. Such'anetwork is described in an article by O. J. Zobel in the Bell SystemTechnical Journal for July 1928, pages 438 to 534, and more particularlyas shown in Figs. 8 and 9 on pages 4'70 and 472, respectively. In Fig. 9it will be noted that B1 or 132 exhibits substantially constant valuesover an appreciable range of frequencies, from 0.4 to 0.8 of the scaleof frequencies used. It will therefore be apparent to one skilled in theart how a network such as l or 2 of Fig. 8 could be made to furnish thecharacteristics desired over any predetermined frequency range.

When an operator at the distant end of the line L2 connects thereto thesignaling current of critical frequency, the output current from networkI'! is shifted 180 degrees in phase from the input current. However, dueto the reversal of current over the conductors I8 and 19 this phaseshift is nullified andthe current delivered to the left winding oftransformer 2| is accordingly in phase with the current delivered to theleft winding of transformer 20. At a phase shift of 180 degrees and itsnullification by reversal it follows that the potentials applied by theright windings'of transformers 29 and 2! to the grid elements 28 and 29,respectively, are substantially in phase; consequently, when from thelocal alternating current source 21 a positive halfcycle occurs whichmatches a condition of simultaneous positive potential on grid elements28. and 29, then tubes 22 and 22 are simultaneously ionized and acurrent flows through the winding of relay 26, which operates and closesan obvious circuit through the line lamp 30, which lights. The circuitthrough the tubes may be traced from current source 21, through windingof relay 26, anode element 3!, tube 22, cathode element 32, conductor25, anode element 33, tube 23, cathode element 3 3 to current source 21.In order to assure that the positive half cycle from source 21 willquickly coincide with the proper grid potential for the condition ofsimultaneous ionization of tubes 22 and 23, the frequency of the currentof source 21 differs from that of the critical signaling frequency ofline L2. In practice it has been found satisfactory to use a linesignaling frequency of nominally 1000 cycles per second while thefrequency of the current from source 21 is nominally 800 cycles persecend. The use of currents of different frequencies avoids thepossibility of a delayed signal if currents of approximately the samefrequency were used and these currents were in step and differed 180 inphase.

In order to apply an approximately equal amount of breakdown potentialacross the oathode-anode circuits of tubes 22 and 23, respectively, theequalizing shunt resistances 35 and 36 are used. These resistances arehigh in value, each being rated at approximately a half megohm. By usinghigh resistances, the effect of either tube remaining conducting duringa positive halfcycle of voltage from source 21, after the grid potentialhas dropped below the critical value required for ionization, isavoided. When a smgle tube only conducts under this condition the anodecurrent is so small due to the high resistance 35 or 36 that the gridregains control as soon as it becomes more negative than the criticalpotential. This feature avoids the possibility that both tubes 22 and 23might remain simultaneously ionized during a positive half-cycle fromsource 21, even though the grid potentials applied by the right windingsof transformers 20 and 2! were sufficiently out-of-phase to effectnon-simultaneous ionization and thereby properly indicate anon-signaling condition. Unless the resistances 35 and 3'6 aresufficiently high in magnitude so that the grid circuit can regaincontrol as before described, it is necessary to use a frequency forcurrent source 21, which is approximately of the same order of magnitudeas that of the incoming line signaling current. On the basis of afrequency of 1000 cycles for the line current, a frequency of 800 cycleswas assumed for source 21. With this frequency the time of the positivehalf-cycle of current from source 2'! is not very much greater than thatof the corresponding half cycles of grid potential from line L2.Consequently the average current through the series anode circuit oftubes 22 and 23 is not much affected by either tube remaining ionizedafter its grid potential is reduced below the critical value. This isdue to the fact, that when the potentials applied to the grids of tubes22 and 23 are out-of-phase under a non-signaling condition', thisdifference of phase measured in time is aproximately of the same orderof magnitude as the time of a positive half-cycle of current from source21. However, by using high resistances, as described hereinbefore, acurrent source of comparatively low frequency such as 60 cycle lightingcurrent may be conveniently used.

In the arrangements of Figs. 1 and 2 gas-filled ionic tubes of the hotcathode type are shown. It is to be understood, however, that ionictubes of the cold cathode double gap may be used instead of the hotcathode type without departing basically from the arrangements of theinvention that have been shown. If the cold cathode type of tube isused, the control cathode takes the place of the grid element of the hotcathode tube. This type of tube is described in U. S. Patent No.2,004,244, issued June 11, 1935 to W. H. T. Holden.

The anode supply as shown in the arrangements of Figs. 1 and 2 is froman alternating current source. It is to be understood, however, that anoscillatory type of anode circuit, sup plied from a continuous currentsource may be used. This would effect a periodic extinguishing of theare through the tube in the same manner as from an alternating currentsupply, and would permit a very high frequency of anode interruption tobe used.

What is claimed is:

1. In an alternating current selective signal receiving arrangement, asignal device, means comprising a gas-filled discharge device forcontrolling said signal device, a source of alternating currentcomprisng a line, connections between said line and the input and outputcircuits of said discharge device and a phase shifting network includedin the connection between the line and the input circuit of thedischarge device so constructed that its attenuation is substantiallyindependent of all frequencies transmitted over said line and to causethe displacement of phase of waves within a predetermined critical rangeof said transmitted frequencies by 180 degrees with respect to the waveswithin the same critical 75 range supplied to the output circuit of thesame device.

2. In a selective signaling system, a. transmission line, two signalreceiving circuits associated therewith comprising a gas-filleddischarge means having an input and output circuit, a connection betweensaid line and both said input and output circuits, a phase shiftingnetwork included in one of said connections and so constructed andarranged that its attenuation is substantially independent of allfrequencies within the range transmitted over said line and that analternating current wave of a predetermined critical frequency withinsaid range transmitted over said line will be applied to one of saidsignal circuits at an angle of 180 degrees with respect to the waveapplied to the other circuit, said output circuit includingelectromagnetic relay means and a signal device controlled by said relaymeans.

3. In a selective signaling system, a transmission line, a signalreceiving circuit associated therewith and including a gas-filleddischarge device having an input circuit and an output circuit, circuitmeans including an amplifier for supplying anode current from said lineto said discharge device, circuit means including a phase shiftingnetwork for supplying input potential from said line to said device,said network being so constructed that its attenuation is substantiallyindependent of all frequencies within the range transmitted over saidline and that an alternating current wave flowing in said line andthrough said network at a predetermined critical frequency within saidrange will be displaced 180 degrees, a signal and a circuit therefor, aguard relay energized from said line at all frequencies flowing therein,said guard relay being arranged to partially close said signal circuitand a relay in the output circuit of said discharge device 2,lli3,1128

adapted to complete said signal circuit when no current flows in saidoutput circuit.

4. In an alternating current selective signal receiving arrangement, a.signal device, a gasfilled discharge tube, a source of alternatingcurrent comprising a line, a connection between the line and the inputcircuit of said tube, a phase shifting network included in saidconnection, said network being so constructed and arranged that itsattenuation, is substantially independent of all frequencies within therange transmitted over said line and that a wave of predeterminedcritical frequency within said range transmitted therethrough will bedisplaced 180 degrees from the angle of the applied wave, and means alsocontrolled by said line current without phase displacement, said meanscooperating. with said tube to operate said signal device when thefrequency of the line current lies within a predetermined criticalrange.

5. In analternatingcurrent selective signal receiving arrangement, aline having two branches, a phase shifting network included in one ofsaid branches so constructed and arranged that its attenuation to thefrequencies applied thereto is substantially independent of frequencyand that within a predetermined critical range of said frequencies theapplied wave will be displaced by 180 degrees with respect to the wavesof the same frequency in the other branch, an electron discharge devicesupplied by each branch and a circuit, including a signal device,associated with the output circuit of said discharge devices in such amanner that the circuit will be closed to operate said signal deviceonly when the input circuits of said discharge devices have apredetermined phase relation.

ALBERT E. BACHELET. WILLIAM H. T. HOLDEN.

